The transmission of video signals over a relatively narrow bandwidth channel at a relatively slow rate, known commonly as facsimile transmission, can with sufficient coding and decoding apparatus preserve both point-to-point detail and point-to-point intensity variations of picture information. If the picture information is sensed by a scanning device having an unenclosed target area, room ambient light can fall upon the object to be scanned. Since the intensity of the room lighting is generally not uniform throughout the room and is also subject to change, exposure of a document or other graphic material to this lighting often produces unwanted amplitude variations in the image signal. These variations may be considered as distortions since amplitude variations in the image signal should relate only to visual details of graphic material being scanned. Amplitude distortion of the signal from the image scanning device also results from certain characteristics of the optical path components such as the lens, and self-contained lighting souces.
Image scanner systems generally operate in a digital fashion in which one or more bits are used to represent a picture element. The electronic decision-making process that determines the presentation of a picture element representing a region, as, for example, either black or white, is begun by scanning an image scene whose signal amplitudes represent the intensities of shade areas in the image. When the intensity of these regions exceed an arbitrary threshold, they are transmitted as signals of a first state, e.g., "on" pulses. Those regions whose intensities fail to exceed the threshold are transmitted as signals of a second state, e.g., "off" pulses. A simple fixed threshold, however, is generally not sufficient to adequately define all the regions of the images of graphic materials.
Methods of signal correction in image scanners are known. Some scanners achieve an improvement in image quality by employing shaped thresholds for determining picture detail. Other methods involve tailoring the signal so that a linear threshold may become the basis for comparison. Both methods, however, may neglect one or more of the following degadations: The effects of nonuniform lighting, lens vignetting, and the resulting change in modulation transfer function values.
In view of the foregoing, it is desirable to have an image scanner adaptable to changes in room ambient light conditions such that it automaically adjusts for changes in brightness and signal levels. It is also desirable to have an image scanner that compensates for lens vignetting and thereby uniformly defines the information regions on a document.